The present invention generally relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus of an image forming system for eliminating a deviation of a diameter of a light spot from a design value.
An optical scanning apparatus of an image forming system such as a laser beam printer is known. In the optical scanning apparatus, a laser light ray emitted from a laser light source is deflected by a rotary deflector, and a deflected light ray is focused on a recording medium as a light spot by means of an optical focusing system. By the rotating movement of the rotary deflector, the light ray deflected from the deflector moves in a main scanning direction so that the recording medium is scanned by the light spot along a main scanning line to record an image.
For recent optical scanning apparatuses, the capability of recording an image with higher dot density is needed. In order to realize the recording of an image with higher dot density, it is necessary to make the diameter of the light spot on the recording medium as small as possible. The diameter of the light spot focused on the recording medium is dependent on the optical parts of the optical scanning apparatus arranged on the optical path from the light source to the scanned surface of the recording medium. Thus, it is necessary that the optical parts of the optical scanning apparatus are designed and assembled in a manner such that a light spot having a diameter as near a design value as possible is formed on the recording medium.
However, it is unavoidable that the optical parts of the optical scanning apparatus have the production errors and the assembly errors. The diameter of the light spot on the recording medium is considerably influenced by the production errors of the optical parts and the assembly errors thereof. Therefore, in order to obtain a desired light spot diameter, some optical scanning devices including a mechanism for adjusting the diameter of the light spot on the recording medium have been proposed.
Japanese Laid-Open Patent Application No. 63-316816 discloses a proposed mechanism for adjusting the diameter of the light spot by using a zoom optical system. However, the cost of the proposed mechanism is considerably high, and it is difficult to make the proposed mechanism viable for optical scanning devices for practical uses.
Japanese Laid-Open Patent Application No. 2-196209 discloses a proposed mechanism for changing the diameter of an aperture through which a light ray from a light source is passed. The proposed mechanism requires a complicated structure to change the diameter of the aperture, and it is difficult to make the proposed mechanism viable for optical scanning devices for practical uses.
In addition, a recent optical scanning apparatus includes a cylinder lens for converting a collimated light ray from the light source into a convergent light ray converging only in a sub-scanning direction so that a line image extending in a main scanning direction is formed on a reflection surface of the rotary deflector. The cylinder lens is provided within the optical scanning apparatus for eliminating the inclination of the reflection surface from the intended direction. However, it is unavoidable that the optical scanning apparatus is provided with an optical focusing system, such as an f.THETA. lens, which has a curvature of field. The curvature of field of the optical focusing system influences the diameter of the light spot focused on the recording medium, and the light spot diameter often varies in both the main scanning direction and the sub-scanning direction due to the curvature of the field. For most of the known optical scanning devices, it is difficult to correct the light spot diameter in both the main scanning direction and the sub-scanning direction.
In order to achieve the recording of an image with higher dot density, it is necessary for the optical scanning apparatus to eliminate the variations of the light spot diameter in both the main scanning direction and the sub-scanning direction due to the curvature of the field of the optical focusing system. An electrical control of a writing signal for recording a dot to the recording medium can be used to correct the variation of the light spot diameter in the main scanning direction. However, the electrical control mentioned above is not effective for correcting the variation of the light spot diameter in the sub-scanning direction.
In order for correcting the diameter of the light spot in the sub-scanning direction which diameter may vary due to the curvature of the field of the optical focusing system, a mechanism for moving the cylinder lens relative to the light source in the optical path direction in accordance with the curvature of the field concerning the sub-scanning direction has been proposed. However, the weight of the cylinder lens is considerably great when a high-speed image recording is performed, and it is difficult for the proposed mechanism to accurately move the cylinder lens relative to the light source at a high speed so as to eliminate the variation of the light spot diameter in the sub-scanning direction.